As materials for steel parts constituting rolling bearings used for automobiles and industrial machines, high-carbon chromium bearing steel defined in JIS-SUJ2 is most frequently used. In general, an important property of bearing steel is a long rolling contact fatigue life, and a possible main factor which influences the rolling contact fatigue life is a non-metallic inclusion in steel. Therefore, as a commonly employed countermeasure, the oxygen content in the high-carbon chromium steel is decreased to control the amount, shape, and size of a non-metallic inclusion, thereby improving a bearing life (refer to, for example, Japanese Unexamined Patent Application Publication No. 1-306542 and Japanese Unexamined Patent Application Publication No. 3-126839).
However, to produce bearing steel containing a small amount of non-metallic inclusion, it is necessary to install expensive refining equipment or significantly improve conventional equipment. Therefore, there is the problem of a high economic load.
Accordingly, research was conducted to resolve the problem. As a result, it was found that even when the amount of a non-metallic conclusion is simply decreased, in many cases, a large effect cannot be obtained on improvement in the rolling life of a bearing, particularly the bearing life under a severe condition such as a high load or a high temperature. This led to the finding that as a factor which determines the rolling life, there is a factor other than the presence of a “non-metallic inclusion” which has been conventionally discussed. Specifically, a microstructural change layer composed of a white etched constituent occurs in a lower layer (surface layer) of a contact plane due to shear stress in contact between inner and outer rings and a rolling element of a bearing as the environment of bearing using becomes severe. In addition, the microstructural change layer is gradually grown as the number of cycles increases, and finally spalling occurs by rolling contact fatigue in the microstructural change portion to determine the bearing life. It was also found that the severe environment of bearing using, i.e., a higher plane pressure (reduction in size) and an elevated using temperature, decrease the number of cycles until a microstructural change has occurred, resulting in a significant decrease in the bearing life. Such a decrease in the bearing life in the severe environment of using cannot be sufficiently suppressed only by controlling the amount of a non-metallic inclusion as in related art. Therefore, it is thought to be necessary to retard the microstructural change.
As a countermeasure, bearing steel containing 0.5 to 1.5% by mass of C, over 2.5 to 8.0% by mass of Cr, 0.001 to 0.015% by mass of Sb, 0.002% by mass or less of O, and the balance composed of Fe and inevitable impurities has been proposed, and bearing steel containing these elements and further containing over 0.5 to 2.5% by mass of Si, 0.05 to 2.0% by mass of Mn, 0.05 to 0.5% by mass of Mo, and 0.005 to 0.07% by mass of Al has been developed (refer to Japanese Unexamined Patent Application Publication No. 6-287691).
As a result, the microstructural change due to cyclic load in rolling contact under high load was retarded, and so-called “B50 high-load rolling contact fatigue life (total number of cycles until a white portion of a microstructural change layer spalls at a cumulative failure probability of 50% in a rolling contact fatigue test)” was improved.
However, the environment of bearing using has been recently made severer than that at the time of filing of Japanese Unexamined Patent Application Publication No. 6-287691, and thus the development of steel having a long rolling contact fatigue life has been desired ardently.
Therefore, a steel was developed having a long rolling contact fatigue life as steel capable of further increasing a bearing life even under severe using conditions, the steel having a composition containing 0.7 to 1.1% by mass of C, 0.5 to 2.0% by mass of Si, 0.4 to 2.5% by mass of Mn, 1.6 to 4.0% by mass of Cr, 0.1 to less than 0.5% by mass of Mo, 0.010 to 0.050% by mass of Al, and the balance composed of Fe and inevitable impurities, being subjected to hardening and tempering, and having a microstructure including residual cementite with a grain diameter of 0.05 to 1.5 μm and prior austenite with a grain diameter of 30 μm or less (refer to Japanese Unexamined Patent Application Publication No. 2004-315890).
With that steel, the average grain diameter of residual cementite is properly controlled to retard the microstructural change and increase the n number of cycles until the microstructure spalls. Furthermore, the grain diameter of prior austenite in the microstructure after hardening and tempering is refined to suppress the development of fatigue cracking and further improve the rolling contact fatigue life.
However, when the steel is applied to a component of an actual bearing, a sufficient rolling contact fatigue life may not be exhibited, thereby causing the need for further improvement.